Well, it depends where you want to work, and in what position. Although web design and web development are two different jobs - web design is art as painting, coding is for geeks like us
. But there are places where a designer is using only tools like Photoshop, and implementing the design on the actual page is the job of the coder. This is not a good approach, but it is a common practice also.
CSS itself is not design, it is only a tool to apply the design on a page. Without knowing what is CSS capable of, you will neither be able to apply the design to the site you code, nor will you be able to give the designer css classes to be filled with the artistic part.
So, yes, you will need to learn css, but that won't make you designer.